The present invention relates to a hydrogen absorbing alloy powder for use as a material for electrodes (negative electrodes) of metallic oxide-hydrogen batteries such as nickel-hydrogen batteries, and a process for producing the powder, and more particularly to the surface treatment of a hydrogen absorbing alloy powder.
Hydrogen absorbing alloy electrodes serving as the negative electrodes of nickel-hydrogen batteries are prepared by pulverizing a hydrogen absorbing alloy ingot to obtain a hydrogen absorbing alloy powder, admixing a binder with the powder and shaping the mixture in the form of the electrode. The hydrogen absorbing alloys heretofore developed include those of AB5-type rare earths having a crystal structure of the CaCu5 type, such Mmxe2x80x94Ni alloys, and TiNi2 alloys having a Laves-phase structure of the C14-type or C15-type.
With the nickel-hydrogen batteries having a hydrogen absorbing alloy electrode as the negative electrode, a gas-phase reaction and an electrochemical reaction proceed at the same time on the surface of the hydrogen absorbing alloy owing to the contact of the alloy surface with an alkaline electrolyte. More specifically, in the relationship between the hydrogen pressure and the temperature, hydrogen is absorbed by the alloy, or the alloy desorbs hydrogen (gas-phase reaction). In the voltage-current relationship, on the other hand, application of voltage (charging) permits the alloy to absorb the hydrogen produced by the electrolysis of water, and the delivery of current (discharging) oxidizes hydrogen to form water (electrochemical reaction). The properties of the alloy surface are therefore important in improving the performance of the nickel-hydrogen battery.
Accordingly, to improve the activity of the hydrogen absorbing alloy for use in nickel-hydrogen batteries, it is conventional practice to immerse a hydrogen alloy powder in an aqueous acid solution for surface treatment as disclosed in JP-B-225975/1993, or in an aqueous alkaline solution for surface treatment as disclosed in JP-B-175339/1988. The surface treatment removes an oxide film formed in the surface layer portions of the alloy particles, permitting rare-earth elements (such as La) to dissolve out and forming a nickel- or cobalt-rich layer in the surfaces layer portions of the particles, whereby the alloy is given improved electrochemical catalytic activity.
However, we have found that the conventional surface treatment still fails to afford sufficient activity although forming the nickel- or cobalt-rich layer in the surface layer portions of the alloy particles.
An object of the present invention is to provide a hydrogen absorbing alloy powder having higher activity than conventionally, a process for producing the powder, a hydrogen absorbing alloy electrode wherein the power is used, and a metallic oxide-hydrogen battery comprising the electrode.
In producing a hydrogen absorbing alloy powder of the present invention, a starting hydrogen absorbing alloy powder containing nickel and cobalt is held in a high-temperature hydrogen atmosphere under the conditions of temperature, pressure and time sufficient to reduce oxides formed in a surface layer portion of each of the alloy particles 1, not melting the alloy particles 1 and not permitting the alloy particles to absorb hydrogen, and thereafter surface-treated with an acid or alkaline treating liquid. In this process, the temperature is in the range of 100xc2x0 C. to 900xc2x0 C., the pressure is in the range of 1 atm to 3 atm, and the time is in the range of 30 minutes to 10 hours. The acid treating liquid is, for example, a hydrochloric acid solution. The alkaline treating liquid to be used is at least one aqueous solution selected, for example, from among aqueous solution of KOH, aqueous solution of NaOH and aqueous solution of LiOH.
The hydrogen absorbing alloy powder obtained by the above production process is applied to an electrically conductive substrate and shaped in the form of an electrode to prepare a hydrogen absorbing alloy electrode of the invention.
The oxide film formed in the surface layer portions of the alloy particles 1 in the step of preparing the starting hydrogen absorbing alloy powder is reduced by the high-temperature hydrogen atmosphere (reduction treatment with high-temperature hydrogen) of the above process and thereby converted to a first metal-rich layer 3 which is enriched in metals (nickel and cobalt). Since the temperature, pressure and time for the treatment are adjusted to the respective ranges mentioned, the oxide film is fully reduced without the likelihood of the alloy particles 1 melting or absorbing hydrogen.
The alloy powder is thereafter subjected to a surface treatment with the acid or alkaline treating liquid, whereby oxides of rare-earth elements (such as La), or Al, etc. are allowed to dissolve out from a surface layer portion of the first metal-rich layer 3. A second metal-rich layer 4 further enriched in the metals (nickel and cobalt) is formed in the surface layer portion of the first metal-rich layer 3. The first metal-rich layer 3 is internally studded with relatively small clusters 30 of the metals (nickel and cobalt), while the second metal-rich layer 4 is visually found to be internally studded with many relatively large clusters 40 of the metals (nickel and cobalt).
According to the present invention, the first metal-rich layer 3 formed by the reduction treatment with high-temperature hydrogen and enriched in nickel and cobalt is treated with an acid or alkali to form the second metal-rich layer 4 which is further enriched in nickel and cobalt. The invention therefore affords higher activity than the prior art wherein an acid treatment or alkali treatment only is conducted.
The hydrogen absorbing alloys usable according to the invention are those having a crystal structure of the CaCu5 type, and alloys having a Laves-phase structure of the C14-type or C15-type. Preferable to use are alloys having a crystal structure of the CaCu5 type.
Examples of useful alloys having the CaCu5-type crystal structure are those represented by MmNi2CoAlMn and obtained by substituting the La of LaNi5 with Mm (misch metal) which is a mixture of rare-earth elements, i.e., alloys represented by the formula MmNixM1yM2z (wherein Mm is a mixture of rare-earth elements, M1 is at least one element selected from among Co, Al and Mn, M2 is a transition metal different from M1, x is a positive real number, x, y and z are such that 4.7xe2x89xa6x+y+zxe2x89xa65.4).
Examples of useful alloys having a Laves-phase structure are those represented by AB2 (wherein A is at least one of Ti and Zr, and B is at least one element selected from among Ni, Co, V, Mn, Fe and Cr). More specifically, TiNi2 and Ti0.5Zr0.5Ni2 are useful.